Direct detection of analytes has many drawbacks, as many analytes are not directly detectable by current techniques. Indirect methods, such as fluorescent or radioactive labeling, are possible, but also have drawbacks. For example, not all analytes can be labeled efficiently, and radioactivity presents safety and licensing concerns.
Other indirect detection methods using either fluorescence or UV-absorbance offer alternate solutions to fluorescent or radioactive labeling. These methods employ strongly UV absorbing/fluorescent markers which are added to the entire background buffer to yield a background signal that is uniform in the absence of analyte ions. Non-fluorescent/non-UV absorbing analyte species are then injected into a separation channel and separated via capillary zone electrophoresis (CZE). As analytes migrate, separate, and disperse, they locally displace the background marker as per the requirements of electroneutrality and current conservation. The displacement of detectable marker ions by undetected analyte ions leads to a reduction in the background signal at the analyte peak location. This local decrease in the background signal is an indirect detection of the analyte zone. These traditional indirect detection methods, however, offer no preconcentration ability, and are therefore usually limited to analyte concentrations above about 0.1 mM. This fairly low sensitivity technique is also susceptible to false positive identifications due to the presence of so-called false system peaks arising in part from disturbances in the background ion distribution created by sample injection. Changes in the background signal due to Joule heating or unstable illumination can also cause false peaks in these indirect detection techniques. Accordingly, there is a need in the art to develop new, more sensitive methods of indirectly detecting analytes that have lower false positive rates.